For a high market penetration of solid state light sources, such as Light Emitting Diode (LED) light sources, at the time being, retrofit lamps are considered quite important. For most consumer luminaires the user/customer may be able to replace only the sole light source (e.g. the incandescent light bulb) inside a luminaire with a modern LED light source at low initial cost. Not only the compatibility with the socket but also the compatibility with the existing installation/circuitry is quite important. Therefore, the light source should fit into existing sockets and should be compatible with the existing electrical installation, including dimmers. This includes interoperability with existing wall dimmers.
Especially Triode for Alternating Current (TRIAC)-based leading edge dimmers are used in many households. With these dimmers, there is quite often the problem that the load of the LED light source is too small to work properly with the dimmer. As a result the LED light source may start to flicker or even permanently switch off when the dimming level becomes too low. That is, in case wall dimmers are present, the low power consumption of the solid state light source may cause problems with the latching and holding current of the TRIAC. Especially with low wattage lamps with a power consumption below 40 W (e.g. a 2 W candle light bulb for a E14 screw socket), the minimum load of the TRIAC dimmer circuit is not reached. Hence, the luminaire will not work at all or might work in a chaotic mode of flashes.
There have been proposals to use a special switch mode power supply to shape the current consumption of the lamp according to the required holding current of the TRIAC. Taking the typical 50 mA of a standard TRIAC, the minimum load would be more then 10 W in a 230V-system. But, for the intended power range of the retrofit lamps, this might still be too much power.
U.S. Pat. No. 7,075,251 discloses a ballast having a resonant feedback circuit drawing continuous input current from a wide range of source voltages to satisfy requirements of phase control dimmers by using a keep-alive functionality. During the time when the input voltage is lower than a bus capacitor voltage, there would be no input current, resulting in the TRIAC going to the off state. To prevent this, a high frequency current is continuously consumed. As a result the TRIAC is kept in the on state, but the load still has to draw the TRIAC holding current.